The present invention relates in general to electrochromic layer-sets, especially those operating with hydrogen. Such layer sets are also referred to as electrochromic layered packages as well as electrochromic composites or laminates. A conventional species of such layer-sets is an electrochromic mirror.
Electrochromic layer-sets, when based on hydrogen ion transport, have generally comprised, a transparent substrate plate being arranged on the front face of the layer-set, a transparent electrode, at least one electrochromic layer (EC-layer), a hydrogen-storing layer, a hydrogen ion-conducting layer, at least one additional electrode, and a rear face which directly follows one of the two electrodes.
Electrochromic materials are materials which change their optical constants (n, k) in response to the application of an electrical field, and which, after the field is switched off, maintain the changed optical state. Moreover, the changed state can be easily reverted to the initial state by reversing the polarity of the electric field. In essence, the electrochromic material is involved in a reversible redox-process.
Typical examples for electrochromic materials are WO.sub.3 and MoO.sub.3, which, when applied in a thin film to a glass carrier, are colorless and transparent. If, however, a sufficient voltage is applied to such a layer, the latter being positioned between other suitable layers, cations, e.g., hydrogen ions, diffuse from one side and electrons from the other side into this layer, thus forming the blue tungsten- and molybdenum-bronze, H.sub.x WO.sub.3 and H.sub.x MoO.sub.3, respectively. This case constitutes a field-controlled system; however, diffusion-controlled systems also exist, in which e.g., hydrogen ions (protons) diffuse directly into the WO.sub.3 -layer from a platinum layer lying behind the WO.sub.3 -layer. The intensity of the coloration is determined by the amount of charge passed into the layer (field-controlled system) or the number of the hydrogen atoms diffused into the layer (diffusion-controlled system).
Layer-sets manufactured by co-application of electrochromic materials can be used to control and vary optical characteristics, particularly light absorption. Such layer-sets are of considerable interest for use as displays as well as for transparent optical equipment, e.g., spectacles and light valves, as well as for reflecting systems, e.g., mirrors and reflecting displays.
A variety of possible constructions of electrochromical layer-sets by different layer arrangements is described, e.g., in Schott Information 1983, no. 1, page 11, in DE-PS 30 08 768, in Chemistry in Britain, 21 (1985), 643 or in Dechema-Monographien, Volume 102--VCH Verlagsgesellschaft 1986, page 483.
In U.S. Pat. No. 4,465,339, electrochromic mirrors are described as being exclusively constructed from solid layers, thereby gaining certain advantages compared to electrochromic mirrors having liquid electrolytes (described, e.g., in U.S. Pat. No. 3,844,636). Such advantages include, for example, e.g., lower thickness of the entire system, no leaking-out of the acid used as electrolyte of the layer-set is fractured.
It is clear from the prior art that a wide variety of different possibilities exist for the arrangement of the individual layers for the construction of an electrochromic mirror. Consequently, the following sequence of layers (in the direction of view) represents only one example thereof:
glass substrate PA0 transparent electrode PA0 electrochromic layer PA0 solid, hydrogen ion-conducting layer PA0 hydrogen-ion permeable reflector PA0 solid, hydrogen ion-conducting layer PA0 hydrogen ion-storing layer PA0 catalytic layer, simultaneously capable of functioning as an electrode PA0 adhesive PA0 stop plate (also called backing, back face, face pane, sealing face or sealing plate)
If the reflection of the mirror is desired to be decreased, the absorption of the EC-layer is increased by intensifying the color. This is accomplished by switching on the circuit wherein the transparent electrode is connected as the cathode and the electrode disposed behind the hydrogen ion-storing layer is connected as the anode. Hydrogen ions move from the hydrogen ion-storing layer through intermediate layers--said layers being hydrogen-ion permeable and electron impermeable--into the electrochromic layer; and electrons pass directly into the EC-layer from the voltage source via the transparent electrode. In the EC-layer a redox-reaction with the electrochromic material, e.g., WO.sub.3, takes place thus forming the blue tungsten-bronze H.sub.x WO.sub.3 according to the following formula: ##STR1## wherein x represents the extent of reaction, which in turn determines the extent of light absorption of the EC-layer.
Since the electrochromic reaction is reversible, the reaction can be reversed by reversing the polarity of the electric field applied to darken the EC-layer, thereby decolorizing the EC-layer to the extent desired. The electrode arranged directly in front of the back face is switched on to be the cathode, causing the hydrogen ions to be transported back into the hydrogen ion-storing layer in reverse of equation (1), as follows: ##STR2##
It is possible during the operation according to equation (2) that protons may reach the cathode where therey are transformed to hydrogen molecules.
At the rear, the system is sealed by a bonded back face made of glass or of bonded or foamed synthetic. The seam between the electrode and the rear face is critical because this seam should prevent the escape of the discharged hydrogen molecules or resultant water molecules after reaction of the hydrogen with oxygen. A major disadvantage of prior electrochromic laminates is that hydrogen is slowly consumed through leakage, the latter being attributable to the prior art constructions or manufacturing technique.
Since the transport of charges necessary for the electrochromic reaction is inseparably linked to mass transfer (in this case by the H.sup.+ -ions) in order to maintain neutrality, the life of an electrochromic layer-set is inversely dependent on the rate of leakage of hydrogen.
A further impairment of the function of such an EC-layer-set results from the fact that the hydrogen of the layer-set can react (deflagrate) by reacting with ambient oxygen diffusing from the edge through the glued joint, causing the zone of functioning plane of the EC-layer-set to decrease from the edge.
In U.S. Pat. No. 4,465,339, col. 5, lines 27-34, metal face panes, as well as metal solders, are mentioned as possible hermetic seals. However, if the electrochromic layer-set is sealed by a soldered metal plate, it is necessary, on information and belief, to use a solder with a melting point below 100.degree. C., because higher temperatures can be detrimental for the electrochromic layer-set. Under most conditions, e.g., in a car, those solders are unsuitable in view of their low melting point. Conversely, if the metal plate is glued, oxygen can diffuse across the glue-layer.